1. Field of the Invention
The present invention relates to an image forming apparatus having plural photoconductors.
2. Description of the Related Art
There has been known an image forming apparatus, i.e., a so-called tandem type image forming apparatus, in which plural toner images are formed by means of plural photoconductors, each corresponding to each toner image, with an electrophotographic process, and these toner images are superimposed. In a tandem type image forming apparatus that forms a full-color image, toner images of respective color components, such as yellow (Y), magenta (M), cyan (C), and black (K), are formed by means of different photoconductors, and each of the toner images is superimposed (see, for example, Japanese Unexamined Patent Application No. 11-91205).
In the tandem type image forming apparatus, it is necessary to drive the plural photoconductors, each corresponding to each toner image, and an image forming section for forming toner images onto the corresponding photoconductors. The number of components can be reduced by driving the photoconductors of Y, M, and C, which are simultaneously driven, and the corresponding image forming sections (including a developing unit) with a single motor in order to reduce the number of components in a drive section so as to downsize the apparatus. On the other hand, as for the black color, the K photoconductor and the K image forming section (including a K developing unit) are driven with a motor different from the motor used for the YMC, since the sections involved with the black color solely form an image during the formation of a monochromatic image. A stepping motor can be used, for example, as a motor for driving the photoconductors of the respective colors and the corresponding image forming sections. However, it is preferable to use a DC motor, which has a driving force per volume greater than that of the stepping motor, in order to drive a great number of loads, such as the loads for the YMC, with a single motor.
In a structure in which each of the photoconductors of the respective colors and the corresponding image forming sections are independently driven, there may be a case in which a capacity of the K developing unit is set to be greater than the capacities of the developing units for the other colors in order to make a frequency of an exchange of the K developing unit equal to that of the developing units for the other colors, since the K developing unit is more frequently used for the monochromatic printing than the other colors. In this case, a DC motor having a great driving force is preferable. A DC motor may sometimes be used for the other colors in order to share a control circuit and a control program with K. However, the problems described below arise when the DC motor is used for the drive.
Specifically, each of the photoconductors has a very small eccentricity due to a processing precision or assembling precision of components. This eccentricity produces a speed irregularity, which agrees with the rotating cycle, in a peripheral speed. A banding (periodic occurrence of coarse portions and fine portions) is produced due to the speed irregularity. When the high-density portions (fine portions) and the low-density portions (coarse portions) in the respective toner images are different in case where the toner images having the banding are superimposed, a color misregistration occurs, and this color misregistration is noticeable. In view of this, in order to match the high-density portions and the low-density portions in the respective toner images, the photoconductors are assembled with the rotational phase thereof adjusted. Further, the drive of each of the photoconductors is controlled so as to keep the adjusted rotational phase.
The control of the rotational phase is easy, if a stepping motor is used. However, when a DC motor is used, an increase curve of the speed of each of the YMC photoconductors and an increase curve of the speed of the K photoconductor during the period from when the respective photoconductors are started to when they reach a predetermined process speed might not be matched. This causes either the YMC photoconductors or the K photoconductor to rotate faster. Accordingly, a misregistration occurs in the rotational phases of the YMC photoconductors and the K photoconductor, before the YMC photoconductors or the K photoconductor reach the process speed.
This will be described in more detail. FIG. 10 is a waveform chart illustrating a speed control when photoconductor drums, which are stopped, are started by means of a DC motor serving as a driving source in a conventional image forming apparatus. In FIG. 10, an axis of ordinate indicates a target drive speed and an actual drive speed of the DC motor. An axis of abscissa indicates a time. At the time of starting the motor (time ts), the target value of the drive speed is set to an initial drive speed Vi upon the starting. The target speed is set to gradually assume a higher value with the lapse of time, and linearly increases to an image forming speed (process speed) Vf, which is determined beforehand for the image formation, at a time t4. For example, the process speed is a peripheral speed of the photoconductor, i.e., 225 mm/sec. The diameter of the photoconductor drum is 30 mm, for example.
On the other hand, a transition state of an actual drive speed of the motor is as described below. The motor keeps stopped for a short while after the start of the motor. During this period, an output of a set comparing circuit 33 changes so as to gradually supply high current to the motor, since a misregistration from the target speed increases. Since the time has elapsed from the starting time ts to the time t0 when the motor starts to rotate, the target speed increases more than Vi. Thereafter, the driving force of the motor overcomes a static friction force, so that each motor starts to rotate at the time t0. The rotational speed sharply increases in order to follow the target speed. The drive speed of the K photoconductor reaches the target speed at the time t1. The target speed at this point is Vi that is greater than the initial drive speed Vi. On the other hand, the drive speeds of the YMC photoconductors reach the target speed at a time t2 because a load is heavier than that of the K photoconductor. The target speed at this point is V2. Because of a difference in a drive load between the YMC photoconductors and the K photoconductor, the K photoconductor increases more sharply than the YMC photoconductors. Therefore, the time taken to reach the target speed is different between the K photoconductor and the YMC photoconductors. In FIG. 10, a difference in the rotational phase, i.e., a difference in the rotational angle, occurs between the K photoconductor and the YMC photoconductors by a distance (the product of the speed and the time) corresponding to an area of an internal region (a hatched region) enclosed by lines linking a point where the time is t0 and the target speed is zero, a point where the time is t1 and the target speed is V1, and a point where the time is t2 and the target speed is V2.
In the Japanese Unexamined Patent Application No. 11-91205, the current value applied to each motor upon the starting is set to be lower than the constant current value applied to each motor upon the steady-state operation of the photoconductors and a transporting belt in order to reduce a load exerted to the drive source upon the starting. However, this control does not aim to reduce the misregistration in the rotational phase upon the starting.